This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Our long-term goal is to elucidate the impact of the Mdm2-p53 pathway in vivo on tumor development as a necessary prerequisite to the development of therapeutic protocols for attenuating cancer. p53 functions as a tumor suppressor and exerts this function by activating numerous downstream targets such as p21, BAX, PUMA, GADD45 as a transcription factor1. Mdm2 is a major inhibitor of p53 through physical binding and induction of p53 degradation2. The p53 gene is mutated in more than 50% of tumors, and Mdm2 is highly expressed in a variety of tumors, stressing the importance of the Mdm2-p53 pathway in tumor development 3-6. It should be noted that a significant number of tumors have wild-type p53 with Mdm2 over-expression. Interestingly, some patients with tumors in which p53 gene is mutated and Mdm2 is over-expressed are known to have a poorer prognosis, compared to patients with mutant p53 alone in tumors7, 8. These results suggest that Mdm2 also has p53-independent roles in tumor development. Recently, Mdm2 is found to inhibitory interact with other tumor suppressors such as Rb, PML, and p73, which may contribute to p53-independent roles of Mdm2 in tumor development9-15. One can assume that any proteins that affect Mdm2 function may have an impact on tumor development. Mdm2 Binding Protein (MTBP) was isolated by Boyd MT et al.16 using the yeast two hybrid system with Mdm2 as bait. Over-expression of MTBP causes G1 arrest of cell cycle, but the effect is nullified by simultaneous over-expression of Mdm2. Our working hypothesis is that MTBP plays a critical role in tumor development and metastasis. This hypothesis is based on our preliminary results using a conventional mtbp knockout mouse in which we observed that (1) 30% of mtbp heterozygous mice spontaneously developed tumors within 22 months, and (2) doubly heterozygous knockout mice for mtbp and p53 developed metastatic osteosarcomas. Based on these observations, the focus of this proposal is on the roles of MTBP in osteosarcoma metastasis and the functional characterization of this protein. The specific aims are designed to provide a comprehensive assessment of novel functions of MTBP, especially in osteosarcoma metastasis. Aim 1. To characterize osteosarcomas induced by loss of mtbp. Aim 2. To generate and characterize mice with conditional mtbp and/or p53 knockout alleles, specifically in osteoblasts. Aim 3. To isolate MTBP interacting proteins.